Genes, alleles and monohybrid inheritance
The vocabulary of genetics and how to work out a genetic cross.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 17.1.1Define chromosome, gene and allele, and distinguish genotype from phenotype.
- 17.1.2Use genetic diagrams and Punnett squares to predict the results of monohybrid crosses.
6 minute read
Genes, alleles and monohybrid inheritance
The vocabulary
Getting these precise is most of the battle:
- Chromosome — a thread of DNA, made up of many genes.
- Gene — a length of DNA coding for one protein, and so for a characteristic.
- Allele — a different version of the same gene, e.g. the allele for brown eyes and the allele for blue eyes.
- Genotype — the alleles an organism has, e.g. Bb.
- Phenotype — the observable characteristic, e.g. brown eyes.
- Homozygous — two identical alleles (BB or bb).
- Heterozygous — two different alleles (Bb).
- Dominant — an allele expressed even when only one copy is present (capital letter).
- Recessive — an allele only expressed when two copies are present (lower case).
Working a monohybrid cross
Use a Punnett square and work methodically:
- Write the parents' genotypes.
- Write the gametes each can produce.
- Combine them in the grid.
- Read off the genotype ratio, then convert to a phenotype ratio.
For Bb × Bb: gametes are B and b from each parent. The grid gives BB, Bb, Bb, bb — a genotype ratio of 1:2:1 and a phenotype ratio of 3 brown : 1 blue, since BB and Bb both show the dominant phenotype.
Why recessive conditions can skip generations
Two heterozygous parents (Bb) both show the dominant phenotype but each carry the recessive allele. There is a 1 in 4 chance their child inherits bb and shows the recessive phenotype — which is how a condition can appear in a child whose parents do not have it.
Think of it like this
Alleles are two copies of a recipe. A dominant allele is a recipe shouted loudly enough to drown out the other; the quiet recessive recipe is still there and can be passed on, only being heard when both copies are quiet.
Worked examples
Method, step by step
Two brown-eyed parents, both heterozygous (Bb), have a child. What is the probability the child has blue eyes (bb)?
- 1Parents: Bb × Bb.
- 2Gametes from each parent: B or b.
- 3Punnett square gives BB, Bb, Bb, bb.
- 4Only bb gives blue eyes — that is 1 of the 4 equally likely outcomes.
1 in 4, or 25%.
Common misconceptions
- Using "gene" when you mean "allele". The gene is for eye colour; the allele is the particular version, brown or blue.
- Thinking dominant means "more common". Dominance is about expression when heterozygous, not frequency in a population.
- Reading a 3:1 ratio as a guarantee. It is a probability — four children could easily all show the dominant phenotype.
In the exam
- Always show the full genetic diagram: parental genotypes, gametes (usually circled), the Punnett square, and the offspring ratio. Marks are given for the working, not just the answer.
- Choose sensible letters where the capital and lower case look different — B and b are clear; S and s cause errors when handwritten.